In recent years, with the spread of the compact disk (CD), the superiority and ease of use of digital audio has come to be widely known. It is against this backdrop that the effort to digitalize radio broadcasts has gained momentum. Digital audio broadcasting (DAB) using either satellites or ground stations makes it possible to provide mobile listeners with a high-value-added service. As a result, planar antennae for receiving DAB transmissions which can be made compact and which can be easily mounted have come to be sought.
FIG. 1 is a block diagram of a conventional receiver system. The conventional receiver system shown in FIG. 1 chiefly comprises an antenna unit 2, a cable 3, a receiver unit 4, and amplifier 5 and a speaker 6.
The antenna unit 2, for example, includes a planar antenna and receives polarized radio waves from direction 1. The reception signals received at the antenna unit 2 are supplied to the receiver unit 4 via a cable 3.
The receiver unit 4 demodulates the reception signals supplied from the antenna unit 2 via the cable 3. The signals demodulated at the receiver unit 4 are supplied to the amplifier 5. The amplifier 5 amplifies the signals supplied from the receiver unit 4 and supplies these signals to the speaker 6. The speaker 6 outputs as audio the signals received at the antenna unit 2 and demodulated.
The above-described receiver system 1 only receives predetermined polarized waves, so reception is unstable. With mobile reception in particular, the planar antenna rotates as the signal moves, so without obtaining the necessary polarized waves good reception is difficult.
As a result, in order to maintain good mobile reception it is necessary to mount planar antennae to receive polarized waves from different directions and so selectively receive the polarized waves.
However, mounting a plurality of antenna units both increases the cost of the receiver system and complicates the installation and connection of the antenna unit.
In particular, mobile reception of DAB transmissions requires separate mounting of a left-handed polarized wave reception antenna unit 11 and a right-handed polarized wave reception antenna unit 12, thus making it difficult to make the mobile antenna compact and easy to mount.
Accordingly, there is a method of mounting multiple antenna elements on a single antenna unit. In this sort of configuration, typically, signals providing good reception are selected from among a plurality signals received by a plurality of antenna elements and supplied to the receiver unit. As a result, it is necessary to provide a signal switching circuit for selectively outputting to the antenna unit one signal from among the plurality of signals.
FIG. 2 is a block diagram of a conventional signal switching circuit. The signal switching circuit 10 selectively outputs according to a control signal either an input signal supplied to a first input terminal T1 or an input signal supplied to a second input terminal T2. To make this sort of signal switching circuit 10 a changeover switch 11 is used.
The changeover switch 11 comprises a signal pass-through type switching integrated circuit (IC). In the changeover switch 11, a first input terminal T1 is connected to input terminal Ta, a second input terminal T2 is connected input terminal Tb, an output terminal Tout is connected to output terminal Tc and a control terminal Tcont is connected to control terminal Td. The changeover switch 11 outputs to the output terminal Tout from the output terminal Tc one or the other of either a first input signal supplied to the input terminal Ta from the first input terminal T1 or a second input signal supplied to the input terminal Tb from the second input terminal T2 in response to a switching control signal supplied to the control terminal Td from the control terminal Tcont.
A brief description will now be given of the operation of the signal switching circuit 10.
FIG. 3 is a diagram for explaining the operation of a conventional signal switching circuit. In FIG. 3, (A) is a switching control signal supplied to the control terminal Tcont, (B) is a changeover switch 11 switching state and (C) is an output terminal Tout signal output state.
At a time t10, as indicated by (A) in FIG. 3, the control signal supplied to control terminal Tcont is at level LOW, input terminal Ta of the changeover switch 11 is connected to output terminal Tc and, as indicated by (C) in FIG. 3, a first input signal S1 input by the first input terminal T1 is supplied to the output terminal Tout.
Next, at a time t11, as indicated by (A) in FIG. 3, the control signal supplied to the control terminal Tcont is at a level HIGH, the input terminal Tb of the changeover switch 11 is connected to the output terminal Tc and, as indicated by (C) in FIG. 3, a second input signal S2 input by the second input terminal T2 is supplied to the output terminal Tout.
In order to transmit the output signal in the above-described signal switching circuit, in addition to the signal line a control line for supplying the control signal is necessary, thus complicating the wiring and increasing the cost.
Additionally, since input signal S1 is output via the changeover switch 11, signal loss is also experienced.
The present invention was conceived with the above-described points in mind, and has as its object to provide an antenna unit capable of receiving a plurality of different broadcast transmissions with a single antenna unit and a signal switching circuit that eliminates the need for a control signal line.